Children w/ sickle cell disease demonstrate an elevated resting glomerular filtration rate (rGFR) which decreases w/ age. This may produce an increased workload for each nephron during this period of increased rGFR, which may explain, in part, the high risk for development of renal insufficiency in sickle cell disease. The physiology of this phenomenon is currently unknown. In order to better understand this physiology, it is necessary to measure the workload of the kidney. GFR has long been used as an indicator of kidney function in patients with renal disease. Severity and prognosis of disease are indicated by this parameter. However, proponents of a hyperfiltration theory in renal physiology suggest that glomerular filtration rate is not a fixed entity, but rather a dynamic entity and therefore may not be a good indicator of renal disease. Renal functional reserve (RFR) is a better indicator of workload per nephron and thus is a better indicator of existing renal dysfunction. Very few studies on renal functional researve have been performed in children and has not been studied in sickle cell disease. The cause of the increaase in rGFR of pediatric sickle cell disease is unknown. It is our goal to evaluate renal workload by assessing a parameter other than rGFR: functional renal reserve. In this study we will determine RFR of children with sickle cell disease and unaffected children and statistically compare RFR between the groups. In order to study this we plan to determine RFR by measuring GFR before and after an oral protein load in children with uncomplicated sickle-cell disease, sickle trait, and healthy children. We will obtain urine creatinine volume and serum creatinine from these subjects in order to determine GFR. We will then calculate the change in GFR (or RFR) and statistically compare the renal reserve of the different groups. We expect that the sickle-cell disease group will have lower RFR in comparison to healthy children and children with sickle-trait and that their elevated GFR represents an elevation of the basal/rGFR to a maximal workload capacity without any further compensatory ability to increase total GFR capacity. If, as hyposthesized, there is an increased nephron workload, it may be beneficial to reduce nephron workload in sickle cell patients by dietary means and thereby reduce the rate of progression of renal insufficiency.